Google Docs is an extremely helpful tool for Beta reading, and is used by several writers of my acquaintance.
- It allows commenting on individual selections of text, so it's easy to comment on a particular snippet or section. (It also allows replying to comments, so people can discuss the comments or expand on them.)
- Viewers can make suggested edits, which the document-owner can accept or reject.
- It is extremely helpful that Google Docs can be shared with anybody with an email account; they don't need to belong or sign up to a service.
- Google Docs is high quality, easy to work with, and free.
- You can easily create a Google Doc for the one specific section you want reviewed at the moment. (Merging multiple documents with different reviewers' comments is, alas, not really support in any way.)
This is all under the assumption that you already have, or can obtain on your own, a pool of beta readers.
You can ask friends of yours to beta-read for you. They don't need to be writers themselves - you need reactions, not writing advice. What is important is that you can trust them to be frank -- somebody saying nice things in order to be polite is no help at all.
Writing groups, online forums, critique circles, are all good places to find some friends willing to exchange critiques with you. Participate in the group structure for as long as you find it helpful, but also, stay in touch with people whose feedback you find truly helpful -- a hand-picked group will always be better than a random collection of people on the internet.
In general, both feedback from random internet people and feedback on individual snippets of text, rather than complete works are something you see much more with amateurs than with pros, or even more casual veterans. This isn't a knock on amateurs; it's simply the consequence of experience and ability:
A writer whose been active for a while has usually managed to build up a contact circle, including finding beta-readers they like and find helpful. That means the writer (and probably also the readers they're using) are likely to withdraw from the "public" pool of readers and critiquers. They've found a higher tier, where both the material and the feedback are on a higher level than the average you'll find on the "public" internet.
Critiques and feedback are usually most helpful in full context. It's much easier to see if a scene works, if you know the buildup that's gone into it. It's easier to answer "Does this story work" than "Is this a good premise" (usually: "It might be, if the execution is good"). And, experienced writers have already gotten in the habit of completing drafts -- one of the biggest obstacles for beginners. So, they have full drafts to share, and completing a draft before requesting feedback is easier for them. For all these reasons, veteran writers usually prefer feedback for a whole piece, than for one snippet -- and platforms aimed at snippets will generally have more of an amateur audience.
But the consequence of all this is that random feedback for random snippets of text is very likely to be of extremely low quality. Which is why I'd be leery of seeking such a platform, even if some do exist.
Once you do complete entire drafts, you can find some good online options for critiques. But without even the bar of completing a story, I'd be much more wary.